Near-threshold ?-parameter measurements of state-selected rotational transitions to the v+ = 0 level of normal and ortho-D2

Photoelectron angular distributions in D2 have been measured for the first time with rotational selectivity. The study focuses on the R(2) 5p? and R(0) 5p? Rydberg states, converging on the v+ = 4 vibrational level of the D2 + ion. These Rydberg states autoionize to populate the X2?g + (v + = 0) state of the ion. The angular asymmetry parameter (?) has been determined, across the photon energy region 15.671-15.684 eV, for transitions involving changes in rotational quantum number, between the initial neutral and final ion state, ?N, of +2 and +4. The ?-parameter values obtained for the ?N = +2 transition indicate that the channel is populated via both p- and f-wave photoemission. The ?-parameter value for the R(2) n = 5p? Rydberg state was found to be closer to zero, particularly for the ?N = +4 transition. It is suggested that this near-isotropic emission is associated with the photo-excited D2 molecule rotating before the autoionizing decay has had a chance to occur. Measurements have been made in both normal and ortho-D2, confirming the results and verifying that the resonant features originate in even rotational levels of the D2 molecule.